The present invention is directed to a geophone for monitoring seismological disturbances. More particularly, the present invention is concerned with a magnetohydrodynamic activated geophone of extremely shock resistant characteristics.
In the prior art a variety of seismological sensing apparatus has been proposed for monitoring various forms of disturbances or shock waves. These shock waves may be generated in response to earthquakes, tests of nuclear warheads, or from other conventional sources such as the hydraulic vibrators or conventional explosions employed in the oil industry to locate sub-surface oil deposits.
When an earthquake occurs, a sudden release of accumulated strain results in the propagation of a number of different types of seismological waves. Geophones have previously been employed to measure various parameters associated with earthquakes, such as the velocity of sub-surface movement of waves, the rate of change of the velocity, and the duration of the event. Shear waves, or S-waves, are the primary signal generated by an earthquake. When an extreme disturbance occurs in an homogeneous environment, a spherical front is generated, and a P-wave results. Such a wave is characterized by alternating compression and rarefaction through the sub-surface of the earth, somewhat similar to the structure of sound waves in air. Nuclear blasts in conjunction with underground tests primarily radiate P-waves.
At the interface of the earth with air Rayleigh waves are generated. Such waves are associated with both earthquakes and underground nuclear tests. Love waves are generated primarily from earthquakes, and are generally transverse to the direction of travel of Rayleigh waves. A variety of other complex waveforms resulting from reflection and refraction effects are also known in seismology. A useful discussion of such waves, along with recitation of the possibility of monitoring such waves for purposes of policing a total test ban treaty, is discussed in Scientific American, volume 247, number 4, pages 47-55, October, 1982.
In the prior art a variety of geophones and/or seismometers have been proposed. Essentially known prior art devices include a rigid, generally conically shaped outer casing or enclosure housing an internal element of some form for sensing vibration. A variety of different sensors and/or transducers have been proposed to originate a signal corresponding to seismological vibration. For example, Hayes in U.S. Pat. No. 1,980,993 discloses a sealed chamber in which pneumatic pressure results in the generation of an electrical signal in response to seismological vibration. Bound in U.S. Pat. No. 3,806,909 employs an internal piezoelectric element sensitive to soil stresses for generating a seismological responsive signal. Massa in U.S. Pat. No. 3,360,772 proposes a geophone in which a bilaminar piezoelectric element is suspended across an interior within the geophone housing for sensing vibrations and producing a proportional electrical signal.
The seismometer proposed by Baltosser in U.S. Pat. No. 2,748,370 contemplates the use of an electromagnetic sensor system interiorly of the casing for producing vibration sensing. Ording in U.S. Pat. Nos. 2,712,124 and 2,759,552 also discloses electromagnetic means for generating a proportional electrical signal. Sanderson in U.S. Pat. No. 2,677,270 senses vibration in response to the differential conductivity within a fluid medium as a gaseous bubble confined within a fluid chamber moves about in response to sudden seismological vibration. Other less relevant art known to me includes U.S. Pats. Nos. 2,683,867 and 3,474,405.